In a child day care center database at church, I have used the following
expression in a PaymentQry to determine a past due amount.

PastDue: IIf([DueDate]Date(),Format(20-Nz([AmtPaid],0),"Currency"),"$0.00")
It has worked fine for several years.

The "20" in the expression represents the amount due for each payment.
Everybody's weekly payment was the same ($20.00).

Now we want to change the payment to allow a discount for a second child.
The first child would be $20 / week with siblings being charged $15 / week
each.

My thought was to add a Payment field to the PaymentTable that would be
picked up in the PaymentQry. Then I added a PmtDue text box on the
PaymentFrm. The amount due for each child would be put in the PmtDue text
box and have the PastDue amount based on the PmtDue rather than the $20 in
the query. The above expression is replaced with: PastDue:
IIf([DueDate]Date(),Format(PmtDue-Nz([AmtPaid],0),"Currency"),"$0.00")

So far when I have tried to change the "20" to "PmtDue" I get errors. I
have totals in the from footer showing totals for "PmtDue" "AmtPaid" and
"PastDue" . When I add the "PmtDue" text box on the form I get #errors in
all the totals in the form footer. I got this error even before I added
the new "PmtDue" total in the footer.

Does this make sense? Is there a better way? Where did I go wrong?

Thanks
Hank